Protective circuit



C. E. TORSCH PROTECTIVE CIRCUIT Filed Feb. 27, 1946 INVENTOR zarlafflfch BY W5 W PITT'ORN EY I Patented July 6, 1948 PROTECTIVE CIRCUIT Charles Edward Toi-sch, Lancaster Township,

Lancaster County, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application February 27, 1946, Serial No. 650,712

4 Claims.

The present invention relates in general to television systems employing cathode ray tubes, and more particularly relates to means for preventing failure of some portion of the cathode ray beam deflecting circuit from causing a so-called "beam burn on the luminescent screen of cathode ray tubes having a separate high voltage supply.

Systems are known in the art in which the relatively high potential required for the second anode of the cathode ray tube is obtained by rectification of the voltage surges developed across one winding of the horizontal output transformer during the retrace, or snapback, portion of each linescanning cycle. When a pulse rectifier of this nature is employed, it follows that failure of some portion of the horizontal scanning circuit which acts to cut olf production of these voltage surges will also cut olf the supply of energy to the anode of the cathode ray tube.

Other known television systems, however, make use of a separate high-voltage source for energizing the cathode ray tube anode. r:This separate source, for example, may comprise a radio-frequency oscillator, a low-frequency power unit connected to the supply line, or a D.C. accumulator such as a rotary Wimshurst machine or even a series battery supply. In such an arrangement, it is apparent that failure of some portion of the scanning circuit which acts to cut oi production of the voltage surges normally developed across the deilection circuit inductance will have no direct eiect on the potential of the cathode ray tube anode, since in this case th'e high-voltage output is substantially independent of deflection.

The effect of failure of both horizontal and vertical deection, in a system such as above set forth, is to cause the cathode ray scanning beam to remain stationary, and thus produce a burn, or permanent discoloration, at or near the center of the luminescent screen. In systems employing an extremely high second anode voltage, the stationary beam may even cause a melting ofthe glass wall of the cathode ray tube. The above injurious effects may also be produced if the scanning generators are intentionally or unintentionally disconnected while the separate highvoltage supply is still in operation.

If either, but not both', of the horizontal or vertical scanning circuits is inoperativey then thev coating of the screen. Accordingly, the present invention makes use of this fact in providing for a television system, incorporating a separate high-voltage supply source, in which failure of horizontal, or line, deflection will cause the viewing screen of the cathode ray tube to remain completely dark, while failure of vertical, or eld, deflection (which is extremely infrequent) will produce only a relatively harmless line trace.

The above mode of operation isachieved, in one embodiment of the invention, by heating the filament of the high-voltage rectifier' tube from one or more turns on the horizontal output transformer. Thus, even though the plate of the rectier tube continues to be supplied with energy from a separate power source, nevertheless failure of horizontal deflection will cause the rectifier tube filament to. become cold so that the tube cathode will no longer emit electrons and thus the tube will cease to conduct. Consequently, the beam-accelerating potential of the cathode ray tube will drop to a point where the screen of the cathode ray tube will not be illuminated, and hence no beam burn can occur.

One object of the present invention, therefore, is to prevent a beam burn on the luminescent screen of a cathode ray tube upon failure of the deiiecting power, the second anode potential for the cathode ray tube being supplied from a separate source not directly related to the defiecting circuit.

Another object of the present invention is to provide means for causing the lainent of the high-voltage rectiiier tube to be de-energized at times when the cathode ray scanning beam is not being deflected at line-scanning frequency, and thus prevent voltage from a separate power source from being supplied through the rectifier to the second anode of the cathode ray tube.

Other objects and advantages will be apparent from the following description of a preferred form of the invention and from the drawing, the single figure of which is la circuit diagram of a preferred embodiment of the present invention.

Referring now to the drawing, there is illustrated by the reference character Hl source of potential for energizing the anode of a cathode ray tube. This source may be of any suitable nature, but in the drawing has been illustrated as a radio-frequency power supply unit which may, for example, be similar to that disclosed in U. S. Patent No. 2,374,781, granted May l, 1945, to Otto H. Schade. Since this unit is fully described in such a patent, the details thereof will not be set forth in the present application, but the unit I may include an electron discharge tube I2 having a cathode I3, a control electrode I4, a screen electrode I S, and an anode I8. A condenser 20 by-passes the screen electrode It to ground for radio frequencies.

The control electrode I4 of tube I2 is returned to ground through a path including grid resistor 22 shunted by condenser 24, and a feedback coil 26. The cathode I 3v of tube I2 is grounded, while the anode I8 is connected to a point of higher positive potential than the screen electrode IS. The anode circuit includes a parallel resonant combination including the primary winding 28 of output transformer 3D shuuted by the condenser 32. 'Ihe secondary circuit of output transformer 3i) includes the secondary winding 33, shunted by the capacitance of the circuit which includes the natural coil capacity, stray capacities, and the capacity of a diode rectifier tube, the function of which will be hereinafter described.

yThe arrow through coils 23 and 33 denotes the reactive coupling therebetween. The arrow prassing through coils 26 and 33 designates the reactive coupling between the output circuit, including coil 33, and the feedback coil 2S. The highfrequency voltage developed across the secondary coil 33, in combination circuit, mentioned abov with the capacity of the is rectified by a highvoltage rectifier tube 34. This tube 34 may coinprise a diode, as shown, the anode of 'which is connected `to the high-potential end of the secondary coil 33. The filament or heater 35 of diode 34 (which filament preferably has a lowv thermal inertia, such as possessed by the filament of the RCA S016) is connected to ground through a relatively high load resistor 33, and the voltage developed across this load resistor 33 during operation of the system is applied to the second anode of a cathode ray tube (not shown) through a filter circuit 38. This lter circuit 38 may be of any'suitable type, and, in the example shown, comprises a series inductance 4I] and two shunt capacitors 4.2 and 44.

'For deflecting the electron beam which is developed within the cathode ray tube (not shown) at line-scanning frequency, there is provided a deflection circuit which includes a horizontal, ,or line, power output tube 46. Power tube is adapted to provide, when voltage variations which may be such as indicated in the drawing by the reference character 48 vare applied to the control electrode thereof, a current output of substantially sawtooth configuration to a pair of horizontal deflection coils '.ill through a horizontal output transformer 52.

The horizontal deflection coils 50, in combination With a pair of Vertical, or eld, deflection coils, preferably form part of a yoke assembly encircling the neck of the cathode ray tube (not shown) to the second anode of which the poten-- tial output of the filter circuit 33 is applied.

Transformer 52 is provided with a secondary winding 54 connected to supply current to the deflection coils 53. Across this secondary winding Ellis connected the series combination of a damper tube 53 and a parallel resistance-condenser network 53, 59, the function of these elements being to damp out high-frequency oscillatiohs which are produced in the secondary circuit following the retrace, or snapb-ack, portion of each deflection cycle. The resistance 59 should preferably be adjustable to regulate the bias on the diode damper tube 56.

In accordance with the present invention,

means are provided whereby failure of the power tube 46 to vary the current at normal amplitude through the primary of the transformer 52 at line-scanning frequency Will de-energize the filament 35 of the high-voltage rectifier tube 34, and thus prevent the output of the power supply lo from. being supplied to the anode of the cathode ray tube through the filter circuit '38. To accomplish the above result, the filament 35 of the high-voltage rectifier tube 34 is heated from 'a third winding 6E! on the horizontal output transformer 52. This winding 60 may comprise one or more turns wound independently of the secondary winding 54. It will be apparent that when a cyclically varying current is flowing through the primary winding 62 of transformer 52, current will be induced in both the windings 54 and 33, and the induced current in winding 63 will now through the filament 35 of rectifier tube 34 to heat the filament and thus permit the tube 34 to rectify the output of power supply I0. It will also be apparent that when no current is flowing through the primary winding 62 of transformer 52, then no current will be induced in the Winding 60, and hence the filament 35 will not be heated. Obviously, when filament 35 is cold, rectifier` tube 34 is incapable of conduction, and hence even though the power supply IIl continues to supply energy to the anode of the rectifier tube 34, nevertheless, this energy will not be rectified by the tube, and there will be no output from the filter circuit 38.

in the above connection, it has been found in practice that the negative voltage excursion on the primary winding 62 is the principal factor in determining the temperature of the filament 35 of tube 34. If the deflection coils 50 are opencircuited, for example, the primary winding 62 of transformer `52 presents a higher inductance to the power tube 45, but the rate of change of current,

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decreases. As a result, the area of the negative excursion in the waveform of the voltage on the primary winding 62 is reduced, and the temperature of the filament 35 is lowered. If the rectifier tube 34 is one in which the filament has a sufl-ciently low thermal inertia, and which is designed to possess suitable electron-emitting properties, such a reduction in its temperature will cause tube 34 to cease conducting. If the defiection coils 5f: are short-circuited, however, no voltage will be applied to filament 35, and complete cooling of the latter will occur.

It will thus be seen that the present invention provides means whereby a failure, or an intentional or unintentional disconnection, of the horizontal scanning transformer primary circuit which prevents current from flowing through the primary winding 0f the horizontal output transformer, or a short circuiting of the secondary or yoke circuit, automatically cuts off the second anode potential from the cathode ray tube. An open-circuiting of the secondary or yoke circuit will also produce such an effect in cases where a suitable coupling or switching unit is employed. As stated above, this will prevent a beam burn, permanent or discoloration, of the fluorescent coating of the cathode ray tube which normally results from a condition where the scanning beam remains stationary and the high second anode Voltage continues to be supplied.

While only a single high-voltage rectifier tube has been illustrated and described, it will be clear that any number of additional high-voltage rectier tubes may be employed if desired, and also that these tubes may be connected together in any suitable manner, such, for example, as the cascade arrangement shown in U. S. Patent No. 2,439,223, issued to O. H. Schade, on April 6, 1948. In such an event, energy for heating the filament of each such additional tube employed would be derived in the same manner as is the energy for heating the filament of the illustrated tube 34.

It will also be clear that other types of power supplies may be utilized to provide energy for the anode of the rectifier tube 34 in place of the R.F. unit l0, as long as the output thereof is substantially independent of the operating status of the deflection circuit. It should be further emphasized that the tube 34 may, when D.C. is applied to the plate thereof with proper polarity, act as a switch for disconnecting such D.C. supply from the cathode ray tube anode when normal deiiection is not in effect.

Having thus described my invention, I claim:

1. In a television system of the type having a cathode ray tube wherein an electron beam is developed and then deflected to scan a luminescent screen, and in which the value of the potential applied to said cathode ray tube for accelerating the electron beam is normally independent of the operating status of the means for deflecting said electron beam, the combination of a source 0i relatively high voltage which alternates in polarity, means, including an electron discharge tube having at least a filament and a plate, for rectifying the loi-directional voltage output of said source so that a substantially smooth D.-C. potential may be obtained for accelerating said electron beam, means, including a transformer, for developing energy of substantially sawtooth waveform for deectlng said electron beam, and means for deriving from said transformer energy for heating the filament of said electron discharge tube. so that said filament is heated to render said tube conductive only during operation of said deflecting means.

2. In a television system having a cathode ray tube in which an electron beam is developed and then deflected to scan a luminescent screen in said tube, to thereby eiect the reproduction of an image, a source of substantially smooth D. C. potential for accelerating said electron beam within said cathode ray tube, said D. C. potential source including a rectifier tube having a filament, and means including a transformer for defiecting said electron beam within said tube, the value of said D. C. potential being normally substantially independent of the operating status of said deflecting means, the improvement which comprises preventing a departure from normal operation of said deflecting means from causing said electron beam to burn the luminescent screen of said cathode ray tube, said improvement comprising a connection between the filament of said rectier tube and said transformer so that said lament is heated by energy derived from said transformer, whereby a departure from normal operation of said deflecting means results in a material drop in the value of said D. C. potential.

3. In a television system having a cathode ray tube: the combination of a power supply unit; an electron discharge device having at least a plate and a filament, the plate of said electron discharge device being connected to the output of said power supply unit; means for applying the output of said electron discharge device to the anode of said cathode ray tube; a horizontal scanning circuit including a horizontal power output tube, a pair of horizontal deflection coils, and a transformer coupling said deflection coils to the output of said horizontal power tube; and means for heating the filament of said electron discharge device by energy derived from said transformer during normal operation of said horizontal scanning circuit.

4. In a television system having a cathode ray tube: the combination of a power supply unit; an electron discharge device having at least a plate and a lament, the plate of said electron discharge device being connected to the output of said power supply unit; means for applying the output of said electron discharge device to the second anode 0f said cathode ray tube; a horizontal scanning circuit including a horizontal power output tube, a pair of horizontal deflection coils, and a transformer coupling said deflection coils to the output of said horizontal power tube; a separate low-voltage winding on said transformer; and means for connecting the lament of said electron discharge device to said lowvoltage transformer winding.

CHARLES EDWARD TORSCH.

REFERENCES CITED The following references are of record in the 

